The invention concerns interleaving techniques. These techniques are generally used to reduce the correlation introduced by ‘selective’ filtering inherent to the transmission channel.
The invention applies to any single-carrier transmission system, i.e. any system in which baseband data symbols are transmitted sequentially in time, as opposed to a multi-carrier signal in which N data symbols previously modulated in baseband by the sub-carriers of an orthogonal frequency-division multiplex (OFDM) are transmitted simultaneously to form a multi-carrier signal. Either way, the data transmitted is then modulated by a frequency in the radio-frequency (RF) band. The baseband information is transmitted in the form of data symbols (QAM, QPSK, BPSK, etc. cells) around the zero frequency.
These symbols are distorted by the transmission channel, the effect of which is to filter the signal transmitted, which is defined by a baseband equivalent filter whose impulse response h(t, τ) depends on two variables t and τ, where t represents time and τrepresents the time-delays associated with the coefficients of the filter at time t. The transmission channel, also referred to as the multipath channel, produces correlation of the received symbols in the time domain over a period of the order of the coherence time of the transmission medium. The coherence time is the average value of the time shift necessary to ensure decorrelation of the signal representative of the transmission medium and the time-shifted version of the signal.
This correlation limits the performance of decision circuits in the receiver. It induces packet errors after decisions are made in respect of the transmitted data symbols and after the bits are decoded. These effects are encountered when the environment is a multipath environment and varies slowly or, more generally, if the transmission bit rate of the system is high compared to the Doppler frequency of the transmission medium.
This applies to ultrawideband systems in particular, in which the transmission band above 500 megahertz (MHz) is used for transmission in the {3.1-10.6} gigahertz (GHz) band in an environment that varies little with time at very high bit rates, up to 1 giga bits per second (Gbps). One example of a UWB system is described in the document by R. Fischer, R. Kohno, M. MacLaughlin, M. Wellborn, “DS-UWB Physical Layer Submission to 802.15 Task Group 3a”, reference: IEEEP802.15-04/137r3, July 2004.
This also applies to millimeter band systems, in particular 60 GHz systems, for which the target bit rates proposed by the IEEE 802.15.3c standardization group are much higher than 1 Gbps. These systems are dedicated to deployment in buildings. They are near-field systems in which RF coverage inside buildings at these bit rates is less than 10 meters (m) to 15 m. More generally, this correlation imposes a penalty on any transmission system in which the transmission bit rate or the modulation rate is high compared to the Doppler frequency of the transmission medium.
Another example concerns systems associated with the IEEE 802.16 standard in bands above 11 GHz and below 66 GHz, in which interleaving methods of single-carrier transmission are static for a given transmission mode. These systems are described in the IEEE document Std 802.16-2004, “IEEE Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Fixed Broadband Wireless Access Systems”, June 2004.
One method of removing this correlation interleaves the binary data or the data symbols for transmission using an interleaving depth expressed in time that must be greater than the coherence time of the transmission medium. In practice, the coherence time is deduced from the autocovariance function Φ(Δt) of the signal associated with the transmission medium and corresponds to the time shift Δt necessary for obtaining a statistical autocorrelation coefficient less than a value usually between 0.9 and 0.5 inclusive. An estimate of this value is also given by the reciprocal of the Doppler frequency of the transmission medium. An interleaving depth greater than the coherence time is in practice difficult to obtain in systems with very high bit rates because the latency time that interleaving introduces into the system limits the available bit rate in real time. Data is transmitted in packet mode. Packet size is generally inversely proportional to bit rate, and each packet is transmitted independently. As a result the interleaving depth is limited by the packet size and is usually less than the coherence time of the transmission medium.
Interleaving techniques are therefore applied to the data in a transmission system to decorrelate the received data and to improve the performance of the decision circuits.
At bit level, if the system includes an error corrector coding device, the interleaving techniques applied after coding reduce the received error packet size because the de-interleaving at the input of the decoder disseminates the bits with a high probability of decision errors and the decoder can correct those errors. Interleaving is referred to as bit interleaving when it is applied to coded bits or as scrambling when it is applied to bits extracted directly from the source.
Symbol interleaving relates to a block of symbols with complex values of given size formed of complex (QPSK, x-QAM, BPSK, etc.) signals resulting from a symbol binary coding operation usually referred to as digital modulation. Interleaving can apply different laws for the phase and quadrature components (I,Q components) of the baseband modulated signal.
Interleaving is always applied to the payload data of the transmitter with a static interleaving law for each transmission mode defined by the number of modulation states, the coding, etc. The expression “payload data” refers to transmitted data that carries an information message and not including data dedicated to signaling and identification. In the remainder of the present document, the term “data” designates “payload data”, whether binary or in data symbol form.
All known single-carrier digital radio communications systems use a specific data permutation law that is unique for a given transmission mode.
The UWB-DS-CDMA (Ultra Wide Band Direct Sequence Coded Division Multiple Access) system promulgated in July 2004 in the context of the IEEE 802.15.3a standard for ultrawideband near-field systems does not include symbol interleaving as such. The bit interleaving applied to the coded bits from the source is similar to symbol interleaving if a symbol consists of one bit. In one transmission mode, the system uses BPSK (binary phase-shift keying) modulation resulting from a thresholding function that transforms a bit having values in the binary space {0,1} into a binary symbol having values in a sub-space {−1,1}. This concept makes the binary interleaver used similar to a symbol interleaver. The UWB-DS-CDMA interleaver is of the convolutional type, and its structure is unique for each transmission mode and common to all the transmission modes. This interleaver is described in the document by J. L. Ramsey, “Realization of optimum interleavers”, IEEE Tr. on Inf. Th., Vol. IT-16, May 1970, pp. 338-345. The transmission system is illustrated by the diagram of FIG. 1. The transmitter EM comprises a source data interleaving module ETBB, a channel coding module CC, a puncturing module PE, a bit interleaving module ETB, a transmit filter FL. The transmitter uses ternary spreading codes with values in the subspace {−1,0,1} of size Lc—24, called −24(−1/0/1) codes.
The interleaving law used by the binary interleaver ETB does not vary with the length of the spreading sequence, the modulation chosen, or the spreading sequence for generating a required bit rate. The interleaving process applied to the bits associated with the BPSK symbols for the obligatory mode of the standard is a convolutional interleaving process that generates a fixed and static spread between the symbols for the transmission mode described. However, this convolutional interleaver is not a block interleaver of size K defining a bijective law in the space of integers over the subset I={0, . . . , K−1}. As its name indicates, the convolutional interleaver defined by the Ramsey algorithm is a device for storing data in a buffer memory and interleaving the data by time-shifting the data on each clock pulse by a delay proportional to an individual delay Jo. The spread corresponding to the distance between the indices of the input data at the output of the interleaving module is a multiple of Jo, the size of the individual shift register of the interleaving system, and is strictly less than the product of the number No of branches and this individual delay Jo. The interleaving process is illustrated by the diagram of FIG. 2, which represents the time shift that is a multiple of Jo between the interleaved coded bits bce and the coded bits be before interleaving. Even if interleaving is convolutional, the interleaving pattern is stationary in the broad sense and the law is constant regardless of the time concerned.
More generally, for known single-carrier systems, the interleaving processes used for a given transmission mode are static and stationary in the broad sense. Static interleaving has advantages for estimating the transmission medium because, provided that the transmission medium varies slowly in time compared to the transmission bit rate, it limits the insertion ratio of the training sequences intended to probe the transmission medium. In contrast, interleaving the data on sending it in order to decorrelate the data on reception is of limited efficacy because of the presence of long-term fading caused by the multipath channel varying slowly in time.